Charged particle lithography systems are known in the art, for example from U.S. Pat. No. 6,958,804 in the name of the applicant. This lithography system uses a plurality of electron beamlets to transfer a pattern to the target surface. The pattern data is sent to a modulation device, also referred to as a beamlet blanker array. Herein, the beamlets are modulated, for example by electrostatic deflection of the beamlets to switch selected beamlets on or off. The modulated beamlets are projected onto the surface of a target to be exposed. To enable high speed transfer of the pattern to the target surface, optical transmission of control signals to the modulation device may be used.
To manufacture lithography systems able to perform exposures having smaller critical pattern dimensions with sufficiently high throughput, charged particle systems have been proposed having a very large number of charged particle beamlets. The number of beams in a charged particle system suitable for smaller critical dimensions may be in the order of tens or hundreds of thousands or millions.
For lithography purposes the area in which final projection occurs is typically limited to a single field, and in a charged particle system where the beamlets remain substantially parallel this results in the area of the modulation device being limited to about 27×27 mm. The electrical power requirements of the modulation device are substantial, and the electrical current flowing in the modulation device will generate magnetic fields. In such a small area, the effect of these magnetic fields becomes significant. Any magnetic fields in the area of the modulation device will exert a deflecting force on the electron beamlets passing through the device, and even very small deflections of the beamlets may result in writing errors on the target.